Recent years have seen technical advances such as in portable electronic devices and hybrid vehicles. Thus, there is growing demand for a higher capacity of secondary batteries (in particular, lithium-ion secondary batteries) for use in those devices and vehicles. In current lithium-ion secondary batteries, the development of high-capacity positive electrodes lags behind that of high-capacity negative electrodes. Even high-capacity Li(Ni,Mn,Co)O2-based materials, which have been actively researched and developed, have a capacity of merely about 250 to 300 mAh/g.
Sulfur, which has a theoretical capacity of as high as about 1,670 mAh/g, is one of the promising high-capacity electrode materials. However, elemental sulfur does not contain lithium, and thus lithium or lithium-containing alloy is required for use in the negative electrode, leaving few options for the negative electrode.
Lithium sulfide, however, contains lithium, and thus graphite or silicon-containing alloy, for example, can be used in negative electrodes; therefore, lithium sulfide can provide a considerably wider selection for negative electrodes and prevent the risk of short circuit and the like caused by dendrites generated by metal lithium. However, as lithium polysulfide, lithium sulfide flows into the electrolyte during the charge or discharge in the battery system using an organic electrolyte, and migrates into the negative electrode to cause segregation (e.g., Non-patent Literature 1 listed below), making it difficult to demonstrate the inherent high capacity of lithium sulfide. Therefore, to improve the performance of batteries including lithium sulfide as a positive electrode, there is a need for measures such as designing positive electrode layers capable of retaining the flowing lithium polysulfide in the positive electrode, creating electrolytes capable of protecting the negative electrode, and providing alternative solid electrolytes not involving lithium polysulfide flow.
One method for suppressing the flow of lithium polysulfide is to form the bond between sulfur atoms and other elements so that sulfur atoms cannot be released during the Li extraction/insertion reaction. For example, Patent Literature 1, listed below, discloses a method comprising adding FeS2 to Li2S to form a composite, and producing a compound of LixFeySz and the like. However, adding a large amount of other elements increases the formula weight of the electrode active material, and also reduces the relative Li content, thus resulting in a decrease in theoretical capacity. In Patent Literature 1, for example, an equimolar amount of FeS2 is added to Li2S to form a composite in which the Fe content is 17%, and the Li content is 33%, with the theoretical capacity estimated from the Li content being about 320 mAh/g, which is, however, significantly lower than the theoretical capacity (about 1,170 mAh/g) of lithium sulfide. Therefore, the amounts of other elements added must be minimized in the production of high-capacity electrode materials.
Nonetheless, the decrease in the amount of other elements added increases free elemental sulfur, which results in an increase in the proportion of elemental sulfur not contributing to the charge and discharge reaction. When used as other elements to be added, transition metals not only further decrease the electrical conductivity but also decrease the utilization rate of the electrode material. For example, as disclosed in Non-patent Literature 2, listed below, although the decrease in the Fe content of Li2S—FeS2 composite from 161 to 3% increases the theoretical capacity from about 350 mAh/g to about 930 mAh/g, the capacity obtained by the actual charge and discharge decreases from about 250 mAh/g to about 3 mAh/g. To form Fe—S bonds and achieve conductivity, the addition of Fe in an amount of about 10% or less is considered to be sufficient. Thus, the probable reason for the decrease in actual measured capacity is that the added Fe atoms are incorporated into the lithium sulfide crystal lattice and fail to form Fe—S bonds. Specifically, lithium sulfide itself remains mostly unchanged in the process of forming a composite, and the incorporated Fe atoms are present as a byproduct of Li2FeS2 and the like, thereby not contributing to increasing the utilization rate of the composite.